Indirect Exciton–Phonon Dynamics in MoS₂ Revealed by Ultrafast Electron Diffraction

Abstract

Transition metal dichalcogenides layered nano‐crystals are emerging as promising candidates for next‐generation optoelectronic and quantum devices. In such systems, the interaction between excitonic states and atomic vibrations is crucial for many fundamental properties, such as carrier mobilities, quantum coherence loss, and heat dissipation. In particular, to fully exploit their valley‐selective excitations, one has to understand the many‐body exciton physics of zone‐edge states. So far, theoretical and experimental studies have mainly focused on the exciton–phonon dynamics in high‐energy direct excitons involving zone‐center phonons. Here, ultrafast electron diffraction and ab initio calculations are used to investigate the many‐body structural dynamics following nearly‐ resonant excitation of low‐energy indirect excitons in MoS2. By exploiting the large momentum carried by scattered electrons, the excitation of in‐plane K‐ and Q‐ phonon modes are identified with 𝑬′ symmetry as key for the stabilization of indirect excitons generated via near‐infrared light at 1.55 eV, and light is shed on the role of phonon anharmonicity and the ensuing structural evolution of the MoS2 crystal lattice. The results highlight the strong selectivity of phononic excitations directly associated with the specific indirect‐ exciton nature of the wavelength‐dependent electronic transitions triggered in the system.

Light-induced hexatic state in a layered quantum material

“…Ultrafast electron diffraction 51 , 52 allows for the study of phase transitions 23 , 32 – 39 , 41 , 57 and transient phonon populations 60 – 64 on ultrafast timescales. Conducting such experiments in an Ultrafast TEM (Fig.…”

Photoinduced Structural Dynamics of 2H-MoTe2Under Extremely High-Density Excitation Conditions

“…We demonstrated ultrafast time-resolved electron diffraction measurements on an ultrathin 2H-MoTe 2 sample. Ultrafast time-resolved electron diffraction measurements can explore the photoinduced structural dynamics of TMDs. − The electron pulse traveled perpendicular to the ab -plane of the 2H-MoTe 2 sample. The obtained electron diffraction pattern from the sample is shown in Figure a, where six symmetric diffraction spots from the {100}, {110}, and {200} planes are clearly observed.…”

Spatially-indirect and hybrid exciton–exciton interaction in MoS2 homobilayers

“…a considerable amount of background charge carriers may be created that do not effectively relax, playing the role of a reservoir interacting with the exciton population. Another explanation could involve enhanced conversion from direct to indirect exciton species due to the phonon-assisted inter-layer charge transfer [48][49][50]. Phonon populations created by the long pulse heating the sample may thus result in the depletion of the DX-A state towards both the IX-A and DX-B populations (in the uncoupled picture) and, therefore, in the much stronger blueshift of the hIX line.…”